I'm trying to make a voltage-follower using an LM324 opamp and a BC547B transistor. Below is the schematics I use. Vcc is 12VDC and input is DC varying between 0-10V.

enter image description here

I then soldered this on a board as in the below photo:

enter image description here C goes to Vcc, B goes to Vout, E goes to -input(pin2) and also goes to the load. I checked the connections many times.

The load is a resistor in series with an LED which indicates the output voltage.

When I vary input LED is always ON with same intensity. Even I remove the opamp LED still is ON.

What might be wrong here if the schematics is right?

  • 1
    \$\begingroup\$ a load resistor in series with an LED indicates the output current. Check your soldering - you may have a short circuit between c-b on the transistor. \$\endgroup\$ – JIm Dearden Apr 27 '16 at 11:59
  • \$\begingroup\$ Yes C and B was shorted internally. I dunno why I replaced it works now Thanks! \$\endgroup\$ – user16307 Apr 27 '16 at 13:00

From your description, I assume that you are trying to visualice the output voltage variation through the LED light intensity.

That is not an easy thing to do, because the eye's dynamic ranges prevents you from noticing any LED's luminous intensity variation. Your LED is in fact varying it's intensity, but not enough to be noticed ant naked eye.

In respect to removing the opamp: That's because some parasitic current is being injected through the base, making the transistors keep conducting and thus keeping the LED ON.

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